Modbus and BACnet Overview - HMD60 - HMD65

HMD65 User Guide

Document code
Document type
User guide

HMD65 can be accessed with the Modbus and BACnet protocols over the RS-485 interface. The protocol selection and communication settings for either protocol are entered with the DIP switches on the HMD65 component board (see Figure 1).

Figure 1. Modbus and BACnet DIP Switches and Screw Terminals on HMD65 Component Board
DIP switch for setting RS-485 termination (120 Ω resistor) ON/OFF
Screw terminals for RS-485 (Modbus/BACnet) communication
Screw terminals for power supply input wiring (15 … 35 VDC / 16 … 24 VAC)
DIP switches for setting the device MAC address: see Figure 2
DIP switches for setting the communication bitrate (4800 … 115200 bps) and parity (N/E)
DIP switch for selecting either Modbus RTU or BACnet MS/TP mode
LED indicators for RS-485 transmit/receive activity


The Modbus variant used in HMD65 is Modbus RTU. For a list of the Modbus registers available in HMD65, see Modbus Registers.


The BACnet variant used in HMD65 is BACnet MS/TP. For a description of the HMD65 BACnet implementation, see BACnet Reference.

Setting Device MAC Address with DIP Switches

Figure 2. MAC Address DIP Switch Example
DIP switches 32, 8, and 1 set to ON
The MAC address is encoded in eight bit binary form, with each numbered switch representing a single bit. This example shows address 41 selected: DIP switches 32, 8, and 1 (decimal: 41, binary: 00101001) are set to ON.

Modbus and BACnet MAC Address Ranges

The BACnet MS/TP MAC address range for the transmitter is 0 … 127 (master mode only).

The Modbus RTU MAC address range for the transmitter is 1 … 247.

Configuring an address above the range maximum results in the address defaulting back to the maximum address (127 or 247). Addresses below the range minimum default to the minimum address (0 or 1).

Bit Rate and Parity Options

  • The bit rate 4800 is used only for Modbus RTU (use 9600 and above for BACnet MS/TP).
  • If the bit rate DIP switches are all set to OFF (left), the following defaults are used:
    • Modbus RTU: 19200
    • BACnet MS/TP: 38400
  • The parity selection (N/E) only has an effect on Modbus RTU communication.